1)    Someone who made a mistake or was forced and did not pray a specific prayer before the final possible time for that particular prayer, when praying the following prayer, he must pray the Amidah a second time as a supplement. If he forgot to pray the morning services he prays the afternoon services twice, if he forgot the afternoon services, he prays the evening services twice, and if he forgot the evening services, he prays the morning services twice. (Shulchan Aruch and Mishne Berurah 108)

2)    When praying a supplementary prayer, one must intend that the first one should be to fulfil the obligation of the prayer of that time, and the second one should be for the supplement. If he intended the opposite, that the first should be for the supplementary prayer and the second for the obligatory prayer, he fulfils the duty of the obligatory prayer, but he must pray the supplementary prayer a second time. (ibid)